Cistern Design Help

Hello, I have arrived at this forum for some advice. I am a general contractor and not an expert on plumbing or pumps. I am hoping that the experts on this forum will troubleshoot my proposed design which is primarily based on my own research.

Here goes:
This system will be built in Puerto Rico, on the coast, where salt corrosion is high and reliable servicemen are scarce. My ultimate goal is to have a reliable system with good if not constant water pressure.

The system will support a 3 unit, 2 story apartment house. 2 units have one bath, the 3rd unit has 3 baths, all have kitchens. I intend to have a separate cistern for irrigation so I can capture rain water.

There is municipal water available, but the water pressure is inadequate. Water will not push to the second floor at all. Most homes in the area have a plastic 800 gallon cistern with a jet pump. it seems that these systems have a limited life given the salt air. And due to issues with installation most are hard to reprime if stopped.

Since I will be 1500 miles away, I want a very reliable system. So my thought was to use a submersible well pump. I also dont want to be out of water given that the municipal system goes down regularly. Thus the large capacity of the cistern (4000 gal)

My proposed system:

We intend to install an 8'x8'x8' underground cistern fed by the municipal water. The construction will be reinforced concrete sealed with food grade epoxy. I will put a pit in the bottom of the cistern about the size of a drywall bucket. Again, sealed concrete. I will then set a 4" well pipe into the bottom of the pit, set directly into concrete. We will drill numerous holes into the bottom of the pipe for water intake. We will then drop a 4" well pump into the pipe. The cistern will be covered with dirt with only the well pipe sticking out of the ground.

I located a Goulds distributor who sized a 18GS 1.5 hp pump. 230V, single phase is available. They also recommended the BF20 balance flow converter. Based on my reading herein, I have abandonded the idea of the BF20.

At face value, this application will function like a standard well pump design. But given that we are dealing with an 8' deep cistern, and not a deep well, the head pressure will be low. We intend to use 1 1/4" pipe from the pump with about 100' to the ground floor level of the house.

Other considerations include needing a way to automatically shut off the pump if the sistern runs dry (float switch?). Feeding water to the irrigation cistern (which will have its own pump) but only when the irrigation cistern is low. And regulating pressure.(cycle stop valve? bladder tank size?)

Does the sizing of the 18GS 1.5 pump seem correct? Does this seem like a reasonable way to proceed? I like the idea of a submersible to avoid priming and corrosion issues (correct assumption?).

The system will support a 3 unit, 2 story apartment house. 2 units have one bath, the 3rd unit has 3 baths, all have kitchens. I intend to have a separate cistern for irrigation so I can capture rain water.

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What type of fixtures in the bathrooms; any two person showers, any body sprays, large jetted or non jetted tubs?

There is municipal water available, but the water pressure is inadequate. Water will not push to the second floor at all. Most homes in the area have a plastic 800 gallon cistern with a jet pump. it seems that these systems have a limited life given the salt air. And due to issues with installation most are hard to reprime if stopped.

Since I will be 1500 miles away, I want a very reliable system. So my thought was to use a submersible well pump. I also dont want to be out of water given that the municipal system goes down regularly. Thus the large capacity of the cistern (4000 gal)

My proposed system:

We intend to install an 8'x8'x8' underground cistern fed by the municipal water. The construction will be reinforced concrete sealed with food grade epoxy. I will put a pit in the bottom of the cistern about the size of a drywall bucket. Again, sealed concrete. I will then set a 4" well pipe into the bottom of the pit, set directly into concrete. We will drill numerous holes into the bottom of the pipe for water intake. We will then drop a 4" well pump into the pipe. The cistern will be covered with dirt with only the well pipe sticking out of the ground.

I located a Goulds distributor who sized a 18GS 1.5 hp pump. 230V, single phase is available. They also recommended the BF20 balance flow converter. Based on my reading herein, I have abandonded the idea of the BF20.

Click to expand...

I don't like the idea of the bucket sized hole in the bottom of the cistern. It will collect sediment. You will eventually have to clean any cistern, so you need to plan for access into it. I don't like the idea of the well casing pipe. Just hanging a pump in the cistern is much easier and less cost etc.. And yes you need float switches to control the pump from running out of water and a float valve to allow automatic refilling of the cistern with muni water as used.

You can hang the pump in like a manhole on top of the cistern off of a 1.25" union to a brass nipple out of a 90* brass elbow and a piece of sch 40 PVC to the pump with a 1.25" male adapters on each end of the PVC. Insulate it to keep vibration noise to a minimum and use a rope etc. to be able to lift it up to get the union undone and to lift the pump 8'. Or use a pitless adapter and then a piece of pipe like we use to pull sub pumps with in a real well.

At face value, this application will function like a standard well pump design. But given that we are dealing with an 8' deep cistern, and not a deep well, the head pressure will be low. We intend to use 1 1/4" pipe from the pump with about 100' to the ground floor level of the house.

Click to expand...

Is there any elevation in the 100'? This 1.25" line, will it have couplers every so many feet, like 20' or be one piece of PE pipe with fittings on only each end (as I would strongly suggest)?

Does the sizing of the 18GS 1.5 pump seem correct? Does this seem like a reasonable way to proceed? I like the idea of a submersible to avoid priming and corrosion issues (correct assumption?).

Click to expand...

No one can say, including my buddy BobNH, until you find out the maximum gpm the apartment house will require and what psi you are going to operate the pump at. And then if this pump is going to fill the irrigation rain water cistern when needed as the folks in the apartment may be using water.

I suspect the well driller went over all of what I am asking above but maybe not fully; especially if he didn't ask what type bathroom fixtures, the maximum number of people in the whole apartment and the age of any children (independent water users). For 60 psi, you need an average of 60 psi so the pump would operate at 50/70 psi; even with the CSV or slightly higher. With the CSV, you can use a small pressure tank and IMO it should be in the building.

And last but nowhere least in importance is water treatment equipment. I have sold to PR and suggest you need some type of disinfection equipment. Chlorine is good in an automatic erosion pellet type chlorination system including a special mixing tank equal to a 120 gal retention tank (you may need more retention/mixing time) but UV is easier to maintain but...

UV will not do anything for any odor created by the storage of 4000 gallons of basically stagnant water in the tropics. Chlorine will. And then we remove it with an automatically backwashed special carbon filter. You may or may not need a water softener too.

You need space for the equipment. Usually a space along a wall 6'-8' and out from the wall 24" with head space of 6-7.5' will allow for all the equipment you would need. You must have a drain for the equipment that can handle upwards of say 100 gallons over 10-12 minutes out of no larger than a 3/4" line from the equipment so the drain line would be at least 1"-1.5 or 2" out of the building etc..

Wow, you guys are thorough. Thanks Gary, I have been out of town and hadnt checked back in a bit. I need to read your post several times to make sure I have got it.

The run from the cistern to the house will be flat and may have one 90 degree turn. I had thought about the sediment issue with the bucket size pit, but I wanted to get the pump as low as possible to further avoid the dry well condition.